Physicochemical and Conformational Studies on BSA-Surfactant Interaction in Aqueous Medium

In this paper, results of physicochemical studies on the interaction of bovine serum albumin (BSA) with alkyltiimethylammonium bromide (ATAB), pentaethylene glycol mono-n-dodecyl ether (C12E5), and sodium dodecyl sulfate (SDS) under the experimental conditions of phosphate buffer at pH 7 in the presence of 10 mM sodium bromide (NaBr), maintaining the ionic strength of the overall solution at mu = 0.015 M, have been presented and discussed. Here, BSA-ATAB corresponds to a polyion-surfactant system bearing opposite charges. BSA precipitated out of the solution on addition of ATAB solution over a certain range of ATAB concentration, the concentration range being dependent on the particular member of the ATAB family. In our earlier reports on the precipitation of oppositely charged polymer-surfactant, the tensiometric profile for surfactant addition in polymer solution differed significantly from that expected from addition of surfactant in the dispersion medium. In the present study, the precipitation process could hardly affect the smoothness of the tensiometric profile. This indicates the interaction process is operative in bulk solution. Microcalorimetric profiles also evidenced an extra hump in the interaction profile at lower surfactant concentrations, without much affecting the dilution enthalpograms beyond mticellization. This interaction appeared unimodal and the extent of interaction increased with increasing tail length of ATAB, evidencing the hydrophobic effect to be an important factor. Addition of salt (NaBr) also affected the nature of interaction: at lower concentration of NaBr, the interaction was mildly assisted, whereas 50 mM NaBr fairly assisted the interaction. The nonionic surfactant C12E5 modestly interacted with BSA. The anionic amphiphile SDS, on the other hand, interacted with BSA in two distinctly different stages, as evidenced from the tensiometric profile. The complexity of the BSA-SDS tensiometric isotherm compared to that of BSA-ATAB arose from the presence of cationic binding sites adjacent to hydrophobic patches of BSA in its native state, so that electrostatic and hydrophobic interactions can cooperatively operate side by side. The interfacial saturation occurred at a lower concentration in the presence of BSA compared to the normal cmc of SDS under identical solution conditions in the absence of BSA, which was slightly delayed for nonionic C12E5. The multitechnique approach evidenced that different experimental techniques probe different physicochemical phenomena and an attempt to show the concurrence of the break points in different techniques is only diluting the essence of this area.